Microstructures in phase inversion membranes . Part 2 .

Membranes were prepared from a castmg solution of a water-soluble polymer, poly (vmyl pyrrohdone) (PVP), and a membrane forming polymer, poly (ether sulfone), m l-methyl-2-pyrrohdone (NMP) as solvent by immersing them m mixtures of water and NMP It was found that the addltlon of PVP to the ternary system suppresses the formatlon of macrovolds m the sub-layer, while the ultraflltratlon-type top-layer consists of a closely packed layer of nodules Using a model for mass transfer m this quaternary system, it 1s possible to explam the effects of the additive on macrovold formation Strong mdlcatlons are found that the appearance of a nodular structure m the top-layer follows a mechanism of spmodal decomposltlon dunng the very early stages of the lmmerslon step